1. Field of the Invention
The present invention relates to a solution of a fluorinated polymer. More particularly, it relates to a solution of a fluorinated polymer having carboxylic acid salt groups in a highly polar organic solvent.
2. Description of the Invention
Fluorinated polymers having carboxylic acid groups such as copolymers of a fluorinated olefin e.g. tetrafluoroethylene or hexafluoropropylene and a monomer having the formula EQU CF.sub.2 .dbd.CF--OCF.sub.2 CFX).sub.l (O).sub.m (CFY).sub.n A
wherein X and Y respectively represents F or C.sub.1 -C.sub.12 fluoroalkyl group; and A reprsents carboxylic acid group or a precursor thereof; l is 0 to 3, m is 0 or 1 and n is 1 to 12 have been used as cation exchange resins having excellent oxidation resistance, chlorine resistance, alkali resistance and heat resistance. These copolymers have been used as, for example, a membrane for an electrolysis for producing an alkali metal hydroxide and chlorine, a membrane for fuel cell and a membrane for dialysis.
If it is possible to obtain a solution of a fluorinated polymer having carboxylic acid groups, it is easy to fabricate a membrane by a simple process and to obtain a substrate having complicated shape or a membrane having thin thickness and to prepare a membrane impregnating said solution into a base and it is further possible to repair pin-holes in a membrane or to coat a substrate with said fluorinated polymer, advantageously.
However, a solubility of a fluorinated polymer having carboxylic acid groups has not been known and it has not been known to dissolve a fluorinated polymer having carboxylic acid groups.
Fluorinated polymers such as tetrafluoroethylene polymer tetrafluoroethylene/hexafluoropropylene copolymer and tetrafluoroethylene/perfluoro(alkylvinyl ether) copolymer are less soluble in comparison with hydrocarbon polymers and are insoluble in most of liquid media.
However, it has been known that certain fluorinated polymers having strong acidic groups such as sulfonic acid groups in a form of sulfamide or sulfonate groups may be dissolved in special polar organic solvents as disclosed in Japanese Patent Publication No. 13333/1973.
However, a fluorinated polymer having carboxylic acid groups are not soluble in the solvent for dissolving the fluorinated polymer having sulfonic acid groups, because carboxylic acid groups as side chains in the polymer chain impart different characteristics.
In the examples of Japanese Patent Publication No. 13333/1973, there is the disclosure that the copolymer of tetrafluoroethylene and perfluoro(3,6-dioxa-4-methyl-7-octensulfonic acid) (17 mole % or 19 mole % of a content of the monomer having sulfonic acid group) in an acid from can be dissolved in n-butanol or dimethylacetamide.
However, corresponding fluorinated polymers having carboxylic acid groups instead of sulfonic acid groups could not be dissolved in these solvents.